Of transporter design, screening for gating modifiers, and how TRAAK gates

This month's installment of Generally Physiological considers how to design a transporter, an approach to scree n­ ing for drugs that target the Na v voltage­sensing domain IV paddle motif, and how the mechanosen­ sitive TRAAK channel is gated by membrane tension. The ability to design an artificial protein with functional activity has exciting implications for biomedical engineering, and can also substantiate our understanding of the mecha­ nisms through which natural proteins act and of the relationship between amino acid sequence and protein structure. Whereas substantial prog­ ress has been made in the design of water­soluble and catalytically active proteins, membrane proteins have presented more of a challenge (see Lupas, 2014). Noting that membrane transporters are hypothesized to undergo a conformational change that allows alternating access of a substrate­binding site to either side of the membrane, Joh et al. (2014) de­ signed a minimal protein based on a four­helix bundle fold that recapitu­ lated this process, transporting Zn 2+ and H + in opposite directions. The engineered protein, called " Rocker " because it was designed to rock be­ tween two alternating states, con­ tained two di­metal binding sites. Rocker selectively transported Zn 2+ and Co 2+ , but not Ca 2+ , and was capa­ ble of using the Zn 2+ concentration gradient to transport H + against a pH gradient in a remarkable demonstra­ tion of the ability to design a mem­ brane protein with defined structural and functional properties. Screening for agents that modify Na v gating Voltage­gated so­ dium (Na v) chan­ nels play a key role in action poten­ tial propagation and thus provide an enticing target for toxins from various venomous creatures. Indeed, different classes of toxins have evolved that bind to different sites on Na v channels and, consequently, have different effects on chan­ nel function. For instance, ­scorpion toxins that tar­ get the Na v voltage­sensing domain IV (VSD IV) paddle motif inhibit fast inactivation to prolong action poten­ tial duration, whereas toxins targeting the VSD I–III paddle motif typically disrupt channel activation. Con­ versely, agents that target specific sites on Na v channels, such as the VSD IV paddle motif, could potentially be beneficial under pathophysiologi­ cal con ditions that involve aberrant channel activity. However, identifying such compounds is a nontrivial task. In this issue, Martin­Euclaire et al. used surface plasmon resonance to investigate the pharmacological sen­ sitivity of the isolated …